Work scheduling of centralized electricity generation, such as from electricity power plants, is known. Such work scheduling includes, e.g., scheduling of discharge and curtailment events. However, known solutions are poorly applicable for determining optimal schedules for distributed energy resources, such as distributed consumer electrical power generation devices and distributed power storage devices such as batteries in consumer power control appliances. Such distributed energy resources are described in U.S. patent application Ser. No. 11/968,941 entitled “Utility Console for Controlling Aggregated Energy Resources” filed Jan. 3, 2008, which is incorporated herein by reference in its entirety. Known solutions for scheduling discharge and curtailment events are particularly inapplicable to distributed energy resources where the quantity of such resources is relatively large and where the discharge capability varies for each unit.
One distributed energy resource is plug-in electric vehicles (“PEVs”). A PEV is any vehicle such as a car, truck, bus, motorcycle, etc that draws electricity from a power distribution network (“grid”), stores the electricity through some means, and uses electricity to power the vehicle. A PEV may come in a variety of forms, including hybridized drivetrain and electric-only drivetrain vehicles.
Hybridized drivetrain vehicles use a combination of electricity drawn from the grid and on-board motive force that may be used to both drive the vehicle and/or as a generation source to extend the range of the vehicle by augmenting the on-board electricity storage. The on-board motive force/generation source can include a variety of power plants including gasoline, diesel, bio-fuel combustion engines driving a generator. Or the on-board electricity generation may come from more advanced means such as fuel cells that use hydrogen, or other fuels to generate a flow of electricity. In the future, it is possible that some part of the electricity generation will come from photo-voltaic generation, kinetic energy capture, or advanced technology means. In general, most hybridized drivetrains generate additional electricity for on-board storage through regeneration by using the motor as a generator during coasting and braking operations.
Electric-only drivetrain vehicles use only an electric motor(s) to provide motive force coupled with sufficient electricity storage to provide suitable driving characteristics and range. As with the hybridized drivetrain, the energy storage may be in a variety of forms: chemical batteries, electrostatic capacitive storage, or a combination of the two. Other forms of energy storage may include electro-kinetic such as flywheels, or thermal methods that rely upon the energy captured and released during phase-change operations. The electric-only drivetrain may use regeneration (see above) to capture electricity for storage to extend the range of the vehicle. In addition, there is the potential to use extra-vehicular means to generate or transfer electricity into the car for direct motive force or to supplement the energy storage. Examples of this include magneto-coupling built into roadways, linear generators embedded into roadways, or other means not yet contemplated that involve interaction between the vehicle and its environment.
The amount of electricity storage on the vehicle varies as to whether it is a hybridized or an all-electric configuration. Current development efforts by the automotive community indicate that a hybridized drivetrain requires 12-16 kWh of on-board energy storage and that all electric vehicles will require 50-60 kWh of energy storage, depending upon desired range and performance characteristics. The primary limiting factors of the storage capacity remain both physical size, weight, and cost of the storage medium. The secondary limiting factors will be crashworthiness, replenishment times, and electrical infrastructure within the home or at commercial charging stations. As new materials and methods come to market, the on-board storage capacity will increase over time with the significant possibility that an all-electric drivetrain will be prevalent in the daily transportation vehicles on the road.
While the PEV has tremendous consumer and societal benefits, it potentially has a significant negative impact on electric grid operations. This is due to the charging requirements of the vehicle and innate consumer behavior. For example, a PEV that has 16 kWh of energy storage that is depleted 80% every day will require 12.8 kWh of replenishment before use again the next day. A typical 110V wall outlet of 20 amp capacity—with many only at 15 amps—limits the current draw to roughly 2000 watts. Charge management algorithms for chemical batteries are non-linear with a decrease in current flow into the batteries when they are both near empty and near full. As such, the charge time is extended beyond the six hours normally expected in this case if the charging cycle was linear. The amount of “stretch” required for optimal charge management varies by battery type and manufacturer.
The combination of the high draw rate (2000 watts), the time required (6-8 hours) to replenish the stored energy, and the timing of the consumer places a significant burden on the electric power delivery system when millions of PEVs are on the road. Once the energy storage device is in “bulk charge” mode—neither almost empty nor almost full—it is drawing current at a 100% duty cycle. This is unlike any other major consumption item within most households except lighting, which generally accounts for a relatively small percentage of electricity consumption.
Consumer driving habits factor into the problem as well. Assuming that PEVs are used as commuter vehicles, then the typical driving pattern is to unplug in the morning, drive 30-50 miles per day round trip, and then come home between 6 pm and 7 pm to plug the vehicle back into the grid for replenishment. When compared to the average peak draw of a household over the period of one hour, the PEV at 110V/20 A current flow effectively doubles the consumption of the house during a typical evening peak demand period. This level of consumption is not planned for in the generation or distribution capacity of electric service providers. With as little as a few hundred PEVs on a distribution feeder, there can be significant delivery issues for the electric utility. With as little as a few thousand within a service territory charging at peak, there can be significant issues related to generation capacity.
Electric only drivetrains with 50-60 kWh of storage exacerbate this problem further. Normal daily driving habits will probably not drain the stored energy beyond that expected by the hybridized drivetrain. However, a longer daily use pattern, or long trips will require up to three times the replenishment time at 110V/20 A, which results in up to 18 hours of charge time, which is not practical for most applications. While the circuits to support replenishment can be upgraded to 220V at high current limits, the energy storage characteristics will determine how much current can be flowed into the device without damage. However, the larger the current draw, the larger the problem for effective grid management.